Embodiments of the present invention are directed generally to a method and an apparatus for converting a DC voltage to an AC voltage. More specifically, embodiments of the present invention are directed to methods and apparatus for detecting excessive capacitance in a load when converting DC voltages to AC voltages using inverter circuits in devices such as uninterruptible power supplies (UPS).
The use of uninterruptible power supplies (UPSs) having battery back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems, and other data processing systems is well known. FIG. 1 shows a typical prior art UPS 10 used to provide regulated uninterrupted power. The UPS 10 includes an input filter/surge protector 12, a transfer switch 14, a controller 16, a battery 18, a battery charger 19, an inverter 20, and a DCxe2x80x94DC converter 23. The UPS also includes an input 24 for coupling to an AC power source and an outlet 26 for coupling to a load.
The UPS 10 operates as follows. The filter/surge protector 12 receives input AC power from the AC power source through the input 24, filters the input AC power and provides filtered AC power to the transfer switch and the battery charger. The transfer switch 14 receives the AC power from the filter/surge protector 12 and also receives AC power from the inverter 20. The controller 16 determines whether the AC power available from the filter/surge protector is within predetermined tolerances, and if so, controls the transfer switch to provide the AC power from the filter/surge protector to the outlet 26. If the input AC power to the UPS is not within the predetermined tolerances, which may occur because of xe2x80x9cbrown out,xe2x80x9d xe2x80x9chigh line,xe2x80x9d or xe2x80x9cblack outxe2x80x9d conditions, or due to power surges, then the controller controls the transfer switch to provide the AC power from the inverter 20. The DCxe2x80x94DC converter 23 is an optional component that converts the output of the battery to a voltage that is compatible with the inverter. Depending on the particular inverter and battery used the inverter may be operatively coupled to the battery either directly or through a DCxe2x80x94DC converter.
The inverter 20 of the prior art UPS 10 receives DC power from the DCxe2x80x94DC converter 23, converts the DC voltage to AC voltage, and regulates the AC voltage to predetermined specifications. The inverter 20 provides the regulated AC voltage to the transfer switch. Depending on the capacity of the battery and the power requirements of the load, the UPS 10 can provide power to the load during brief power source xe2x80x9cdropoutsxe2x80x9d or for extended power outages.
In typical medium power, low cost inverters, such as inverter 20 of UPS 10, the waveform of the AC voltage has a rectangular shape rather than a sinusoidal shape. A typical prior art inverter circuit 100 is shown in FIG. 2 coupled to a DC voltage source 18a and coupled to a typical load 126 comprising a load resistor 128 and a load capacitor 130. The DC voltage source 18a may be a battery, or may include a battery 18 coupled to a DCxe2x80x94DC converter 23 and a capacitor 25 as shown in FIG. 2A. Typical loads have a capacitive component due to the presence of an EMI filter in the load. The inverter circuit 100 includes four switches S1, S2, S3 and S4. Each of the switches is implemented using power MOSFET devices which consist of a transistor 106, 112, 118, 124 having an intrinsic diode 104, 110, 116, and 122. Each of the transistors 106, 112, 118 and 124 has a gate, respectively 107, 109, 111 and 113. As understood by those skilled in the art, each of the switches S1-S4 can be controlled using a control signal input to its gate. FIG. 3 provides timing waveforms for the switches to generate an output AC voltage waveform Vout (also shown in FIG. 3) across the capacitor 130 and the resistor 128.
A major drawback for various inverter circuits is that for loads having a capacitive component, a significant amount of power is dissipated as the load capacitance is charged and discharged during each half-cycle of the AC waveform. Part of this power is absorbed by the inverter circuit switches, which generates heat and causes temperature rises in those switches. To dissipate the heat, the switches are mounted on relatively large heat sinks. According to a known method, to better manage the heat dissipation, the inverter circuit is designed around a safe operating maximum capacitive load. However, in the event that a capacitive load greater than the specified load is applied to the inverter circuit, the heat generated by the switches may be greater than the heat dissipated. As a result, excessive heat causes components in the inverter circuit and in particular the switches to get hotter and hotter and eventually, the switches fail. Accordingly, a method and apparatus is required to overcome the shortcomings of above and other shortcomings.
One aspect of the invention is directed to an uninterruptible power supply for providing AC power to a load having a first capacitive element. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, and an inverter operatively coupled to file DC voltage source to receive DC power and to provide AC power. The inverter includes first and second output nodes to provide AC power to the load having the first capacitive element, first and second input nodes to receive DC power from the DC voltage source, a circuit operatively coupled to the first output node of the inverter, the circuit being configured to compare a value representative of load capacitance of the first capacitive element with a reference value to determine excessive load capacitance, a set of switches operatively coupled between the first and second output nodes and the first and second input nodes and controlled to generate AC power from the DC power. The power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.
A second aspect of the invention is directed to an uninterruptible power supply for providing AC power to a load having a first capacitive element. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, and an inverter operatively coupled to the DC voltage source to receive DC power and to provide AC power. The inverter includes first and second output nodes to provide AC power to the load having the first capacitive element, first and second input nodes to receive DC power from the DC voltage source, means for comparing a value representative of load capacitance of the first capacitive element with a reference value to determine excessive load capacitance, a set of switches operatively coupled between the first and second output nodes and the first and second input nodes and controlled to generate AC power from the DC power. The power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.